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Aluminum From Domestic Clay 
Via a Chloride Process 



The State-of-the-Art 



By A. Landsberg 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8923 



Aluminum From Domestic Clay 
Via a Chloride Process 



The State-of-the-Art 



By A. Landsberg 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



Mm 

MO. 8933 



«!P 



m ^^^ 



This publication has been cataloged as follows: 



Landsberg, Arne 

Aluminum from domestic clay via a chloride process. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8923) 

Bibliography: p. 12-15. 

Supt. of Docs, no.: I 28.27:8923. 

1. Aluminum— Metallurgy. 2. K^aolinite. 3. Chlorination. I. Title. 
II. Series: Information circular (United States. Bureau of Mines) ; 89 23. 



-fN^aS^W-^ {TN775] 



622s [669'. 722] 82-600392 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Background 2 

Technical requirements 3 

Aluminum chloride production 4 

Chlorlnatlon of domestic clay 5 

Purification of anhydrous aluminum chloride 10 

Conclusions 12 

References 12 

ILLUSTRATIONS 

1 . Processing routes to aluminum 5 

2 . Generalized process flowsheet 5 

3 . Pertinent vapor pressures 7 

TABLES 

1 . Pertinent compositions 3 

2. Reactions pertinent to clay chlorlnatlon 6 

3. Quantities of materials per pound of aluminum 6 

4. Reactions pertinent to aluminum chloride purification 10 





UNIT OF MEASURE ABBREVIATIONS 


USED IN 


THIS 


REPORT 


atm 


atmosphere 


kcal 




kilocalorie 


Btu 


British thermal unit 


lb 




pound 


° C 


degree Celsius 


pet 




percent 


K 


Kelvin 


yr 




year 



ALUMINUM FROM DOMESTIC CLAY VIA A CHLORIDE PROCESS 

The State-of-the-Art 
By A. Landsberg ^ 




ABSTRACT 

Kaolinitic clays are potentially a vast domestic resource for alumi- 
num. Utilization of this resource could decrease or eliminate the near- 
ly complete dependence of the United States on foreign raw materials for 
this important metal. Furthermore, processing of clay to aluminum 
through anhydrous chloride metallurgy could reduce the high electrical 
energy requirements of the conventional Hall-Heroult aluminum reduction 
process. Several anhydrous chloride processes have been proposed; how- 
ever, unresolved technical problems have prevented their coimnercializa- 
tion. In particular, an acceptable chemical means has not been found to 
extract aluminum from clay as a highly pure anhydrous aluminum chloride. 
This Bureau of Mines report identifies and discusses the important chem- 
ical problems involved in achieving an acceptably rapid, self -heating, 
selective chlorination reaction and the subsequent separation of iron 
chloride byproduct from the anhydrous aluminum chloride. 

— — 

'Chemical engineer, Albany Research Center, Bureau of Mines, Albany, OR. 



INTRODUCTION 



The worldwide use of aluminum exceeds 
that of all other metals except iron. 
Fortunately, aluminum is one of the most 
abundant elements in the earth's crust 
and bauxite, the principal ore of alumi- 
num, is plentiful and accessible to sur- 
face mining. However, the prime bauxite 
deposits are situated in tropical areas 
so that the largest producer and consumer 
of aluminum, the United States, must 
import over 90 pet of its aluminum raw 
material (24). 2 

The present commercial production of 
aluminum is dependent upon two processes: 
the Bayer process for producing pure alu- 
mina from bauxite and the Hall-Heroult 
process for electrolytically reducing the 
alumina to aluminum metal. These two 
processes are very energy intensive, re- 
quiring over 244 million Btu to produce 
1 ton of aluminum. In comparison, only 
24 million Btu are needed to produce 
1 ton of steel (_3 ) . Furthermore, over 
two-thirds of the energy used in the alu- 
minum industry is consumed as expensive 
electrical energy in the Hall-Heroult 
reduction process. Research and develop- 
ment efforts have increased the effi- 
ciency of the Hall-Heroult process since 
its discovery nearly 100 yr ago to the 
point that further major improvements are 
unlikely. 



Producing aluminum from domestic re- 
sources with less electrical energy has 
been the goal of several proposed alumi- 
num processes ( 15 , 37). While a few of 
these schemes have been developed through 
the pilot plant scale, technical and 
economic realities have precluded adop- 
tion of new aluminum technologies. 
Nevertheless, the search continues. The 
success of anhydrous chloride metallurgy 
in the commercial-scale production of the 
reactive metals titanium and zirco- 
nium has provided motivation to develop 
an analogous chloride metallurgy for 
aluminum. Recent operation of pilot- 
plant-scale aluminum chloride reduction 
cells has revived interest in the chlo- 
ride process for aluminum (42). 

However, as is often the case with 
new and promising technology, factual 
information has often been strongly ex- 
ceeded by publicity and speculation, fre- 
quently colored by a lack of technical 
understanding, and sometimes completely 
neglected in an overpowering urge to 
secure the benefits and rewards of a 
more efficient process. Therefore, it 
is timely that a review be made to de- 
scribe the chloride metallurgy of alumi- 
num, to evaluate recent developments, and 
to outline the problems that remain 
unresolved. 



BACKGROUND 



The technical literature abounds with 
suggested methods for making aluminum but 
the Hall-Heroult reduction process con- 
tinues to be the commercial method used 
and alumina to feed the reduction cells 
continues to be made by the Bayer process 
from bauxite (14) . There are good rea- 
sons for this. 

First, the raw material is found in 
large, easily mined deposits that have 
been concentrated and upgraded over geo- 
logic time by the action of water. 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of the report. 



The resulting bauxite contains alumina 
relatively free of impurities. Second, 
the established, large-scale Bayer proc- 
ess uses common reagents to refine this 
bauxite to cell-grade alumina. And fi- 
nally, the purified alumina is reduced to 
aluminum metal in well-engineered Hall- 
Heroult cells. This makes up today's 
aluminum industry: a large-scale, highly 
efficient technology which has enjoyed 
nearly a century of development through 
huge investments of human effort and 
financial resources ( 13 ) . In order to 
be a viable competitor any new alumi- 
num technology must be economically 
superior and an almost instant tech- 
nical success. Nonetheless, with current 



TABLE 1, - Pertinent compositions, percentages 





Kaolinite 
(AI2O3 '28102 •2H2O) 


Typical clay (calcined) 


Jamaican 




Georgia 
kaolin 


Used for 
calculations 


bauxite 
dried' 


AloO:. 


39.5 

46.5 

NAp 

NAp 

14.0 


40 -45 

49 -52 

.5- 3 

1 - 2 

NAp 


45 

51 

2 

2 

NAp 


49 


"~^2^3 ••••••••••••• 

Si02 


2 


Fe90T.. 


20 


TiOo 


2.5 


Loss on ignition. . 


26 



NAp Not applicable, 'Reference 49. 

emphasis on using domestic raw materials 
and with the escalating cost of energy, 
an increasing interest has been shown in 
new aluminum producing technology. 

Among potential domestic resources of 
aluminum, high-alumina clays appear to be 
the most favorable ( 36 , 39 , 50). In par- 
ticular, the kaolinitic clay deposits in 
the southeastern United States are at- 
tractive because of their size, accessi- 
bility, and quality. The richest depos- 
its, found in Georgia, are estimated to 
have resources in excess of 3 billion 
tons accessible to mining and process- 
ing (50). Such clay can be considered 
of high quality because of its high alu- 
mina content and because of its lack of 
complexing impurities. Composition of a 
typical high-quality Georgia clay is 
shown in table 1. 

For processing purposes this clay can 
be considered as a mixture of silica, 
alumina, titania, and iron oxide chem- 
ically separable but not separable by 
mechanical means. The silica in the 



quantities present can be considered a 
waste product, the titania a valuable by- 
product; and the iron a nuisance by- 
product. It is the qualitative differ- 
ences between clay and bauxite that give 
the competitive edge to the latter. 

If political and strategic considera- 
tions dictate the use of clay as a domes- 
tic aluminum source, it is conceivable to 
extract alumina suitable for reduction in 
existing primary aluminum plants (38). 
However, if this new resource is exploit- 
ed it is also conceivable to extract the 
aluminum in a form suitable for reduction 
in a new, more energy efficient process. 
Recent patent disclosures and successful 
large-scale operation of an aluminum 
chloride reduction cell offer such a 
process (9^, 17 , 42 , 44-45) . There now 
exists a potential domestic resource and 
a developing, energy-efficient technology 
for producing aluminum; lacking is a 
proven technology for converting the kao- 
linitic clay into anhydrous aluminum 
chloride suitable for electrolytic reduc- 
tion to metal. 



TECHNICAL REQUIREMENTS 



I 



The nature of kaolinitic clay and the 
requirements for the aluminum chloride 
product should be examined before consid- 
ering the chlorination process. Kao- 
linite, a hydrated aluminum silicate, 
consists of alternate single layers of 
AI2O3 and Si02 held together by the water 
of hydration. On a microscopic level, 
kaolinitic clay is seen to be made up of 
stacks of hexagonal sheets much like 
pages of a book (22). The major physical 
impurity is separate grains of quartz 



sand. The iron and titanium impurities 
appear to be within the kaolin structure 
because they cannot be completely removed 
without chemical destruction of the par- 
ent kaolin. 

The free and structural water associ- 
ated with clay cannot be tolerated in a 
chlorinator. Under the reaction condi- 
tions water and chlorine as a chloride 
combine to give the corresponding oxide 
and hydrogen chloride, a waste product 



that must be scrubbed from the exiting 
gases. For every one part of water near- 
ly four parts of chlorine are thus wasted 
as hydrogen chloride. Therefore, raw 
clay must be dried of free moisture and 
calcined to remove the water of hydra- 
tion. A calcination temperature of 
700° C is required ( 12 , 19). At this and 
higher temperatures, and depending upon 
time, atmosphere, and the presence of 
other substances, kaolinitic clay can be- 
come quite refractory to subsequent chlo- 
rination (25). Therefore, this step in 
clay preparation can be critical to the 
entire chlorination process and should be 
carefully considered. 

The calcination step can also be used 
to fix the form of clay feed. The plas- 
ticity of wet clay and its strength after 
the removal of hydration water allow it 
to be made into any form from fine pow- 
ders to pellets of any size and shape. 
Prior to calcination, catalysts and 
reductants may be added to assure inti- 
mate contact in the final feed. But as a 
good thing can be overdone so can the 
property of clay to make strong fired 
shapes become a disadvantage. If the 
final feed form is made too dense there 
is a reduction in accessible surface area 
where chemical reaction occurs and the 
rate of chlorination becomes exceedingly 
slow. In general, kaolinitic clay can be 
considered an ideal raw material with 
respect to preparation as a reactor feed, 
but the preparation itself must be care- 
fully controlled so that the desirable 
characteristics are not lost before the 
feed enters a reactor. 



It is important that the anhydrous alu- 
minum chloride product of chlorination 
and subsequent purification processes be 
very pure (41) . The aluminum chloride 
will be fed as a vapor or solid into the 
high-temperature, molten-salt electro- 
lytic cell that discharges only re- 
cyclable chlorine gas and product liquid 
aluminum metal. All impurities entering 
with the aluminum chloride must exit with 
these two or build up in the cell, caus- 
ing premature shutdown ( 17 ) . Some impu- 
rities, notably oxides, would react with 
cell components causing deterioration of 
carbon electrodes. 

Of course, small quantities of impuri- 
ties such as iron, silicon, sulfur, phos- 
phorus, and other metals can be removed 
in association with the product aluminum 
but these must be kept below rigid speci- 
fications (O. Likewise some gaseous 
impurities and reaction products such as 
carbon monoxide, carbon dioxide, and sul- 
fur chlorides , can be removed in the 
chlorine stream. In addition, a system 
to circulate the molten salt electrolyte 
and filter out impurities prior to intro- 
duction into the cell, much like the oil 
system in an automobile engine, has been 
described in the patent literature (17) . 
Any such system would, of course, require 
more expensive equipment, add to the com- 
plexity of operation, and increase costs. 
Preparation of feed for electrolytic re- 
duction must keep the impurities to with- 
in a few hundredths of a percent (41) 
while processing aluminum chloride at 
tons per hour. 



ALUMINUM CHLORIDE PRODUCTION 



Several schemes for processing aluminum 
mineral resources to aluminum metal are 
shown in figure 1. At the far left is 
the traditional Bayer-Hall-Heroult com- 
bination; the double line branching from 
this indicates the path taken by the 
recent development and operation of a 
pilot production facility (43) ; and the 
dashed lines represent the possibilities 
of interest here. The latter two show a 
combination of chlorination and purifica- 
tion operations. 



Going from right to left across the 
upper part of figure 1, the degree of 
purification prior to chlorination in- 
creases. At the far left the alumina 
intermediate is of sufficient purity so 
that the chloride produced from it may be 
used directly in reduction cells. The 
center path represents removal of nui- 
sance impurities, such as iron, prior to 
chlorination. Attempts to remove iron 
from bauxite and clay indicate that this 



Mineral resource 



A 



Chemical 
processing 



\ 



\ 




\ 



Purification 

and 
chlorination 
\ 
\ 



\ 



Chlorination 




\ 
\ 



\ 
\ 

\ 
Chlorination 

and 
purification 

/ 
/ 



\ / 

Aluminum 
chloride 



Molten chloride 
electrolysis 



Aluminum metal 
FIGURE 1. - Processing routes to aluminum. 



does not now seem possible short of 
total chemical modification of the mate- 
rial such as in the Bayer process (7^, 
20 , 32-33). Therefore, the following 
discussion will consider the path on 
the far right representing chlorination 
of domestic kaolinitic clay followed by 
purification of the anhydrous aluminum 
chloride product as outlined in the gen- 
eralized flowsheet of figure 2. 

CHLORINATION OF DOMESTIC CLAY 

All of the metal oxides associated with 
domestic kaolinitic clay chlorinate in 
the presence of carbon as noted by the 
thermodynamic data in table 2 (28). (A 
negative value for the Gibbs function 
indicates that a reaction is possible 
at the temperature indicated.) Similar- 
ly, all of the chlorides will react 
with oxygen to give the oxides and free 
chlorine. Thus it would seem to be 
a simple matter to completely chlori- 
rinate the clay, separate the chlo- 
rides by fractional vaporization accord- 
ing to the vapor pressures shown in 



Energy- 



Energy- 



— Catalyst 



■Reductont- 
Chlorine- 



Clay 

i 



Drying and 
preparation 



Calcination 



Chlorination 



Residue 



-Water 



■Water 



Chloride vapors 



and gases 



Reagents 
and/or - 
energy 



Energy 



Exit gases 

,1 



Condensation 



T 



Scrubbing or 
recovery 



Purification 



T 



Reagents 
and/or 
energy 



_^To waste, use, 
or recycle 



AICI3 product 
FIGURE 2. - Generalized process flov/sheet. 



TABLE 2. - Reactions pertinent to clay chlorination 



Reaction 



Enthalpy , 


Gibbs 


^^9 00 K> 


function. 


kcal 


^^9 00 K» 




kcal 


^56.4 


NAp 


-21.5 


-66.1 


40.0 


-61.4 


-83.0 


-70.7 


-67.4 


-73.9 


-89.8 


-154.8 


-71.2 


-115.4 


-51.8 


-64.7 


-60.8 


-54.9 


-74.6 


-47.2 


59.5 


.9 



(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


(8) 


(9) 


(10) 


(11) 



Al203'2Si02'2H20 ^ [AI2O3 + 2Si02] + 2H2O. 



2Si02] 
2Si02] 
2Si02] 
2Si02] 
2Si02] 



3Cl2 + 
3Cl2 + 
3Cl2 + 
3C0C1 2 
7Cl2 + 



1.5C -»■ 2AICI3 + 2Si02 + I.5CO2. 



3C ^ 2AICI3 + 2Si02 + 3C0 

3C0 ->- 2AICI3 + 2Si02 + 3CO2 

-»■ 2AICI3 + 2Si02 + 3C02 

3.5C ->► 2AICI3 + 2SiCl4 + 3.5C02. 



[AI2O3 
[AI2O3 
[AI2O3 
[AI2O3 
[AI2O3 ^ 

Fe203 + 3Cl2 + 1.5C ->- 2FeCl3 + 1.5C02 

Ti02 + 2CI2 + C -»- TiCli^ + CO2 

SiCl^ + O2 ->■ Si02 + 2CI2 

2FeCl3 + 1.5 O2 ->■ Fe203 + 3Cl2 

3SiCli, + 2AI2O3 ->■ 4AICI3 + 3Si02 

Not applicable. ^At 298 K, for the water as vapor. 



NAp 



figure 3, reclaim the chlorine from the 
unwanted chlorides by reaction with oxy- 
gen, and electrowin aluminum chloride to 
the desired metal and recyclable chlo- 
rine (6^). But the realities of the phys- 
ical world are not so simple. These 
realities will be examined with reference 
to each operation, process, and output 
stream in the generalized flowsheet in 
figure 2 and the relative quantities of 
materials listed in table 3. 

Although considerable energy is re- 
quired to remove the water, both free and 
combined, which typically totals some 
1.43 lb per pound of aluminum, clay prep- 
aration, drying, and calcination present 
no problems. As was stated earlier, care 
need only be taken to insure that the 
innate reactivity of the clay is not 



destroyed in these operations. A solid 
carbon reductant and/or suitable catalyst 
can be added prior to calcination. 
Carbon added at this point must be very 
finely divided and evenly distributed be- 
cause its reducing effect appears to oc- 
cur only at the immediate clay-carbon 
interface. 

The chlorination process itself is com- 
plex and several reactions occur simulta- 
neously with the chlorination of the alu- 
minum oxide portion of the clay. It is 
desirable to have this latter reaction 
proceed rapidly and completely, so that 
the reaction is as small as possible, and 
so that the chlorine is entirely consumed 
and need not be separated from the prod- 
uct gases and recycled. Although the 
negative value of the Gibbs function 



TABLE 3. - Quantities of materials per pound of aluminum, pounds 



Free water in wet clay (assumed, 

-14 pet) 0.76 

Combined water 67 

Oxygen associated with aluminum 89 

Reductant for aluminum oxide: 

Carbon, C ^ CO 67 

Carbon, C > CO2 33 

Carbon monoxide, CO -> CO2 1.56 

Chlorine in aluminum chloride 3.94 

Aluminum chloride 4.94 

^Assuming total chlorination. 
■^Assuming no chlorination. 



Silicon tetrachloride-'^ 6.05 

Chlorine in silicon tetrachloride^. 5.05 
Silicon dioxide residue^ 2.14 



Iron 

Iron trichloride ^ 

Chlorine in iron trichloride 

Titanium 

Titanium tetrachloride^ 

Chlorine in titanium tetrachloride^ 



.059 

.17 

.11 

.050 

.20 

.15 




Q r— 71 1 — I I I I 



FeaCI 



Feci. 



Circles indica 
melting points 



]te^ 



50 100 

TEMPERATURE,^ C 

FIGURE 3. - Pertinent vapor pressures. 



500 1,000 



predicts that a reaction is favorable, it 
does not indicate the rate of the reac- 
tion. In order to obtain a sufficiently 
rapid reaction, a temperature of between 
550" and 700° C is necessary (26). 
(Table 2 is based on 900 K or 627° C.) 

In this temperature range, several 
catalysts have been reported to enhance 
the chlorination rate of the aluminum 
portion of clay. These include sodium 
chloride (26) , boron trichloride ( 51 ) , 
and sulfur compounds ( 52 ) . Each of these 
increases the reaction rate of alumina 
but, with the exception of higher concen- 
trations of boron trichloride, they do 
not appear to change the rate of silica 
chlorination. The net effect is there- 
fore an apparent selective chlorination 
of alumina. 

The mechanism by which these catalysts 
work continues to be investigated. It is 
known that sodium chloride and perhaps 



boron chloride combine with some of the 
aluminum chloride product to form a liq- 
uid phase. These liquids are capable of 
dissolving small amounts of the aluminum 
oxide portion of the clay, and it is sus- 
pected that this dissolved aluminum oxide 
chlorinates easily and leaves as the 
volatile chloride. 

The fate of the catalyst as well as its 
effectiveness is of importance, A sodium 
chloride catalyst could remain in large 
part with the unreacted residue. It 
would be necessary to remove this soluble 
salt before disposing of the solid resi- 
due. The sulfur and boron most likely 
would leave the reactor as obnoxious 
gaseous chlorides that require very low 
condensation temperatures (refer to fig- 
ure 3) or efficient chemical scrubbing 
for their removal. If scrubbing is nec- 
essary the catalyst is not likely to be 
available for reuse (_5) . It is also pos- 
sible that any of these catalysts 



would combine with or be adsorbed on the 
condensed aluminum chloride product and 
require special removal procedures. 
Finally, the corrosive nature of the 
catalyst products needs to be considered 
with respect to equipment construction. 
Liquid phase sodium aluminum chloride 
could attack refractories in the re- 
actor and the combination of sulfur 
and chlorine is known to be quite 
corrosive (34). 

The carbon reductant used in the chlo- 
rination reaction must be chosen to ful- 
fill several requirements. First, it 
must be reactive, i.e., it must sustain 
the desired alumina chlorination reaction 
with respect to rate and extent, and it 
should be somewhat selective towards the 
chlorination of alumina over silica. 
Carbon monoxide and phosgene best fill 
these requirements (i.»^e reactions 4 and 
5, table 2; some silica does chlorinate 
but this is not indicated in the 
equation) . 

Solid carbons (reactions 2, 3, and 6) 
promote rapid reactions only if they are 
finely divided or have a high sur- 
face area characteristic of expensive 
activated charcoal (26) . According to 
published data, solid carbon is less 
selective in promoting chlorination of 
the alumina over the silica portion of 
clay (26). However, elemental carbon has 
a higher capacity for reaction with ox- 
ides if the final product is carbon di- 
oxide and therefore less carbon is needed 
(compare reactions 2, 3, and 4). 

When handling and storage are consid- 
ered, solid carbon would be the most 
desirable reductant. Extremely toxic 
carbon monoxide gas would have to be 
stored under pressure, requiring special 
precautions. Phosgene too is toxic but 
can be detected by its odor and immediate 
physiological effects. It has the advan- 
tage of being able to be stored as a 
liquid under reasonable pressure and in- 
corporates both the reducing agent and 
chlorine in one compound. 

The enthalpy of reaction is of con- 
siderable importance when selecting 



the reductant. A chlorinator of the size 
to be used in the aluminum industry must 
be internally heated. External heating 
would require that the outer containment 
vessel be the hottest part of the reactor 
and any chlorinating agent reaching it 
would react violently with the metal at 
the temperatures required. With internal 
heating the outer vessel can be kept as 
cool as desired. 

Heat can be generated within the re- 
actor by feeding hot reactants , burning 
some of the carbon reductant with added 
oxygen, and by the chlorination reaction 
itself. Burning with added oxygen not 
only requires oxygen and more carbon but 
it also dilutes and adds to the volume of 
the product gases. The clay itself can 
be fed hot directly from the calcination 
step. As indicated by the values of 
enthalpy in table 2, only two of the 
important reactions occurring in the 
chlorinator, reactions 3 and 11, are 
endo thermic and should be avoided. The 
carbon monoxide generated in reaction 3 
would most likely be reacted further ac- 
cording to reaction 4. Reaction 11 will 
be discussed later. 

Finally, the purity of the reductant 
need be considered. Hydrogen, sulfur, 
and alkaline metals and their oxides 
associated with some carbon materials 
would be detrimental. These would be 
chlorinated in the reactor leaving a 
sticky residue of the nonvolatile chlo- 
rides or add undesirable vapors to the 
exiting gas, and all would consume valu- 
able chlorine. In this latter case small 
amounts of hydrogen are particularly 
wasteful because only one part of hydro- 
gen combines with 35 parts of chlorine by 
weight to give hydrogen chloride. 

Silica, being the predominant species 
in clay, requires special consideration 
with respect to conservation of chlorine. 
Even with the best selectivity reported 
for the preferential chlorination of the 
alumina fraction of clay, some silicon 
tetrachloride also is produced (26) . 
With no market for the large amount that 
would be a byproduct of clay chlorina- 
tion, disposal of silicon tetrachloride 



would be necessary. This would not be 
easy because it readily hydrolyzes to 
silica and hydrogen chloride, a waste of 
valuable chlorine. Three solutions to 
this problem have been proposed. 

The first and least acceptable is to 
scrub the exit waste gases containing the 
silicon tetrachloride vapor with a caus- 
tic solution, losing the chlorine value 
as salt. Another solution is to react 
the silicon tetrachloride with oxygen to 
give disposable silica and recyclable 
chlorine (27). An efficient, proven 
technology for this option is not now 
available. The third proposal is to re- 
cycle silicon tetrachloride through the 
chlorinator to take advantage of reac- 
tion 11 listed in table 2 (^i, 41). 

In the temperature range considered for 
a clay chlorinator, this reaction may go 
in either direction. At higher tempera- 
tures and higher concentrations of sili- 
con tetrachloride, the reaction favors 
the formation of aluminum chloride. At 
slightly lower temperatures and higher 

■concentrations of aluminum chloride, the 
aluminum chloride will react with silica 
to form silicon tetrachloride and alu- 
mina. Therefore, by proper selection of 
temperature and the amount of silicon 
tetrachloride fed with the incoming chlo- 
rine, it is conceivable to prevent 
further chlorination of silica in the 
chlorinator. Both of the latter methods 
require the recovery of silicon tetra- 
chloride from the gaseous products. 
Condensation to accomplish this separa- 
tion requires that the entire gaseous 
product stream be cooled to below 0° C 
and probably as low as -40° C where the 
vapor pressure for silicon tetrachloride 
is about one-hundredth of an atmosphere. 
Such a recovery system would require 
sizable equipment and considerable 
energy ( 53 ) . 

Even if it were economical to recycle 
silicon tetrachloride, circumstances have 
been found which could prevent its feasi- 
bility. It has been reported that clay 
exposed to silicon tetrachloride vapor 
at high temperatures becomes nonre- 
active with respect to chlorination (26). 



Catalysts such as sodium chloride may re- 
verse this condition ( 31 ) . 

The most attractive solution to the 
silicon tetrachloride problem is to limit 
its formation to a level where recovery 
of the chlorine value associated with 
this byproduct is not necessary. Limit- 
ing silica chlorination can be quite com- 
plex (30). In addition to the reductant 
and catalysts used, the chlorination re- 
actor configuration can influence silicon 
tetrachloride formation ( 29 ) . The opera- 
tion of an ideal fluidized bed chlo- 
rinator illustrates this point. 

The solid feed would consist of clay, a 
solid reductant, and catalyst. Chlorine 
would be the fluidizing gas. Outlet gas- 
es ideally would be only carbon dioxide 
and aluminum chloride vapor. Conse- 
quently, the solids overflow would con- 
sist of excess reductant, catalyst, and 
silica residue, with very little alumina. 
This is also the composition of the bed. 
Even though the alumina may be much more 
reactive than the silica, in this 
arrangement there is a much greater op- 
portunity for the silica to react. 

In contrast to the fluidized bed con- 
cept, a countercurrent reactor in which 
the gas stream flows counter to the sol- 
ids could enhance selective chlorination 
of the alumina. Even though incoming 
chlorine would encounter reacted clay 
consisting primarily of silica, any sili- 
con tetrachloride formed would have an 
opportunity to react with alumina in 
fresh clay as the gas stream left the re- 
actor (reaction 11). On the other hand, 
the countercurrent reactor allows any 
silicon tetrachloride formed to pass over 
yet unreacted clay with the possibility 
of inactivation as mentioned above. 

Clay chlorination in a liquid phase re- 
actor with molten salts as the liquid and 
catalyst has been suggested (18). In 
this case, residual powdery silica from 
the unreacted clay would be difficult to 
separate and remove from such a reactor. 

Whatever type of reactor is considered, 
a solid residue will be formed. Disposal 



10 



of this residue must be considered. 
Ideally the residue would consist of in- 
nocuous silica, but in practice there 
would also be some chlorinated products. 
These, especially any toxic chlorinated 
carbon compounds , would have to be de- 
stroyed or disposed of safely. The 
amounts and nature of such compounds de- 
pend upon the minor constituents of the 
clay, reductant, and catalyst and have 
not as yet been reported. 

PURIFICATION OF ANHYDROUS 
ALUMINUM CHLORIDE 

As it emerges from the chlorinator, 
product aluminum chloride is but a part 
of a mixture of gases and vapors from 
which it must be separated and purified. 
Under the most ideal chlorinator opera- 
tion, represented by reaction 2 in ta- 
ble 2, aluminum chloride vapor would con- 
stitute only slightly more than half of 
the total volume of the product stream. 
Furthermore, at the temperatures neces- 
sary for condensation, aluminum chloride 
forms dimers and complex molecules as 
indicated in reactions 12 and 14 (35) of 
table 4. As such it would represent only 
40 pet of the product volume (and 
pressure) . 

Therefore, according to the vapor pres- 
sures shown in figure 3, to recover by 
condensation 99 pet of the product at a 
total pressure of 1 atm, the temperature 
would have to be lowered to 114° C. This 
is about 75° C below the melting point of 
aluminum chloride. The above exercise in 
stoichiometry brings to light the two 
factors that complicate any process for 
purifying aluminum chloride: the tend- 
ency of aluminum chloride to form complex 
molecules with itself and other chlorides 



and its high vapor pressure at 
ing point (16). 



its melt- 



It would not be practical to recover 
aluminum chloride as a liquid from chlo- 
rinator gases; the total pressure re- 
quired is too high. Instead it would be 
condensed as a fluffy powder unless 
special conditions such as fluidized bed 
condensers were employed. The more 
dense, granular, more easily handled 
product of the fluidized bed condenser 
is preferred ( 23 , 46 ) . Because aluminum 
chloride is hygroscopic, it must not come 
into contact with atmospheric moisture 
and, therefore, any transport or storage 
between the chlorination and purification 
processes requires specially designed 
equipment. A free-flowing material such 
as the product of the fluidized bed would 
be advantageous. 

As a first consideration, controlled 
condensation appears to be the solution 
to obtaining a pure aluminum chloride 
from the chlorination product stream (2). 
According to the simplistic presentation 
of vapor pressures in figure 3, the vapor 
pressures of aluminum chloride and ferric 
chloride differ by a factor of about 
2,000, a ratio near the acceptable limit 
for iron in the aluminum chloride prod- 
uct. Two condensers in series would 
ideally deliver this product. 

The first condenser, operating at about 
190° C, would remove ferric chloride to 
an acceptably low level without condens- 
ing any aluminum chloride. The second 
condenser, held at 100° C, would remove 
essentially all of the aluminum chloride. 
Remaining in the gas exiting the second 
condenser along with the carbon dioxide 
and carbon monoxide, would be titanium 



TABLE 4. - Reactions pertinent to aluminum chloride purification 



Reaction 


Enthalpy , 

^^5 00 K' 
kcal 


Gibbs function, 
kcal 


(12) 2AICI3 ->► AI2CI6 

(13) 2FeCl3 ->■ FeaCle 

(14) AI2CI6 + FeaCle ^ (AlFeCl6)n 

(15) FeCla -y FeCl2 + 0.5 CI2 


-30.0 
-34.2 

±1 

26.9 


-12.8 
-17.2 

±1 

19.1 







11 



tetrachloride and any silicon tetrachlo- 
ride, catalyst products, and unreacted 
chlorine. These chlorides could be con- 
densed at a much lower temperature, sub- 
sequently separated from one another by 
fraction distillation, and used. If not 
recovered they would have to be scrubbed 
and disposed of as waste. There is no 
mention in the technical literature of 
difficulties in separating these lower 
boiling compounds from aluminum chloride; 
however, the separation of chlorides with 
low vapor pressures from aluminum chlo- 
ride is reportedly not as simple as it 
might seem. 

Separation of aluminum and ferric chlo- 
rides is complicated by the formation of 
the mixed dimer AlFeCls according to re- 
action 14 of table 4 (47-48). This 
molecule and multiples of it have been 
reported from mass spectrometer observa- 
tions of the vapor over mixtures of the 
two simple chlorides (11, 21 , 35 , 47 ) . 
Measured properties of the mixed iron- 
aluminum chloride indicate that its vapor 
pressure and the equilibria for reaction 
14 are such that the iron is more inti- 
mately associated with aluminum chloride 
than would be expected from simple vapor 
pressure data. There is a tendency for 
the mixed chloride to form and its vapor 
pressure appears to be closer to that of 
the aluminum chloride dimer than is the 
vapor pressure of ferric chloride. 

Considerable effort has been put into 
developing a scheme to separate iron and 
aluminum chlorides but, notably, success 
of suitable technology for the solution 
to this problem has not been reported in 
the literature. Although fractional dis- 
tillation, fractional sublimation, and 
reduction of ferric chloride to less vol- 
atile ferrous chloride have been sug- 
gested, the operation of a practical 
scheme to separate iron chloride from 
aluminum chloride has not been reported. 

Reducing the iron chloride to the less 
volatile ferrous chloride or iron metal 
followed by separation by volatilization 



of the aluminum chloride may be possible 
(4^) if the appropriate reducing agent can 
be found (8^) . Aluminum metal is a prime 
candidate for the reductant since it 
would not add another material to the 
system, only more aluminum chloride. It 
is, however, an expensive reagent and 
there are technical problems. If solid 
aluminiom is used (i.e., if the reduction 
is carried out below 660° C) large sur- 
face areas of aluminum are needed to con- 
tact the liquid or vapor aluminum 
chloride-iron chloride mixture. This 
surface soon becomes covered with reac- 
tion products and loses its reactivity. 
If the impure aluminum chloride vapor is 
passed through molten aluminum in order 
to reduce and remove the iron impurity, 
high melting iron-aluminum intermetallic 
compounds form. These solids in the 
molten aluminum bath quickly hinder the 
operation. 

Fractional sublimation on a large scale 
has never been a success owing to the 
difficulties inherent in the mechanics of 
moving large quantities of condensing 
solids. In the case of aluminum chloride 
the strict avoidance of contact with the 
atmosphere would only add to these 
difficulties. 

Although other schemes have been pro- 
posed (40) , fractional distillation is 
the most attractive method for separating 
ferric chloride from aluminum chloride. 
The feasibility of this operation depends 
upon the nature of the vapor-liquid equi- 
libria for the binary system, for which 
there are yet no data in the literature. 

Finally, there are minute quantities of 
other elements such as calcium, magne- 
sium, and phosphorus that have not been 
considered. These may present problems 
as impurities which associate with the 
aluminum chloride ( 10 ) . Only extended 
pilot scale operation of an aluminum 
chloride-f rom-clay process could identify 
the importance of these potential 
impurities. 



12 



CONCLUSIONS 



There is a strategic incentive in the 
United States to use domestic clay as a 
resource for aluminum; and there is an 
economic incentive to develop a chloride 
metallurgy to produce aluminum from the 
clay. Together these make the chloride 
route from clay to aluminum attractive 
and worthy of serious consideration. 
However, there are several basic problems 
to be investigated and much development 
work to be done before it can be deter- 
mined if such a tc»chnology can in fact be 
made practical and economical on a large 
industrial scale. Only a sustained, well 
coordinated research and development 
effort can provide the data needed for 
technical and economic evaulations of any 
proposed chloride route for aluminum from 
clay. 

At the present time two areas need to 
be addressed: the clay chlorination 



reaction and the purification of anhy- 
drous aluminum chloride. In particular, 
the chlorination reaction must be under- 
stood so that it can be assessed with 
respect to rapid and complete use of 
chlorine; selectivity of alumina reactiv- 
ity over silica; use of a convenient, 
inexpensive reductant; and heat genera- 
tion to sustain the reaction. 

In addition, the identity and nature of 
product impurities and their interactions 
with anhydrous aluminum chloride must be 
determined. With such information a 
chlorination process and purification 
scheme can be proposed and tested. Only 
then will the technical operation prob- 
lems surface; and only if these can be 
solved with due regard to economic con- 
siderations will the chloride route from 
clay to aluminum become industrially 
possible. 



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15 



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